The present invention relates to a method for the manufacture of fiber-reinforced composite products comprising a matrix composed of a ceramic material, and more particularly to an improved method for making a fiber-reinforced ceramic matrix composite which provides a product of high strength and high density at low cost.
The concept of incorporating fibers or other continuous reinforcing materials or phases in materials subject to tensile stress fracture is quite old, one of the earliest applications being the use of such reinforcement to provide toughened concrete or cement products. For example, U.S. Pat. No. 1,931,494 discloses the use of reinforcing tapes of textile materials to strengthen asbestos cement products. More recently, reinforced cement products have been described in U.S. Pat. No. 4,528,238 which are made by pressing woven or unwoven fiber mats into soft, moldable cement sheet formed by rolling or extrusion, thus to provide fiber-reinforced cement sheet products.
It is also well known to employ fiber or whisker reinforcement to provide toughened refractory ceramic products comprising ceramic matrix materials such as alumina. Thus, for example, U.S. Pat. No. 3,157,222 describes a process for making an alumina ceramic reinforced with fiber wherein a metal fiber or wire is continuously wound onto a substrate while aluminum oxide is applied to the wire and substrate by flame spraying. More recent practice is to apply the ceramic matrix material as a powder to a fibrous reinforcement material, and then to consolidate the coated fibers. Hence U.S. Pat. No. 4,623,228 describes the fabrication of a fiber-reinforced glass composite wherein fibers coated with glass powders are laid up into sheet and consolidated with heat and pressure into a strong, unitary composite.
There also exist applications wherein thin ceramic sheets are employed without fiber reinforcement to provide thin ceramic substrates or barrier layers. U.S. Pat. No. 3,953,703, for example, discloses the manufacture of thin ceramic tape by a process known as tape-casting, wherein a slurry comprising a ceramic powder and a binder is cast or extruded as thin sheet and thereafter dried to provide a thin, relatively tough and flexible ceramic tape. Such tapes have been employed, for example, to provide dielectric layers for capacitors and as thin ceramic substrates for microelectronic devices.
In the field of fiber-reinforced ceramics it is customary to employ flowable ceramic slurries, and to fabricate composite products by coating reinforcing fibers with the slurry and then arranging the coated fibers into the shape of the desired composite. Among the disadvantages of this procedure are the difficulties attendant upon the preparation of slurries having appropriate viscosity and coating characteristics, and the problem of attaining a uniform coating of the ceramic material on the fiber so that a homogeneous composite material may be attained as the coated fibers are formed into the desired product or preform. Therefore, techniques for providing ceramic matrix composites which do not require the slurry-coating of fibers or whiskers, or of felts or woven fabrics made of such fibers, have been sought.
In an alternative approach to the manufacture of such composites, U.S. Pat. No. 4,613,433 teaches a method wherein cloths of fibrous reinforcement material are combined with similarly woven cloths formed of threads or yarns made from the matrix material itself, and the combined cloths consolidated to provide a layered composite structure. Alternatively, threads or yarn of the reinforcing fiber and the matrix material can be woven together into a composite cloth which can be consolidated with heat and pressure to produce a dense fiber-reinforced material.
This method is unfortunately limited in its application to those ceramic matrix materials which can economically and conveniently be formed into thread or yarn. In addition, such procedures risk damage and/or unwanted shifting or the fibers during the consolidation stage of the process, due to the spatial readjustments of matrix and reinforcing fiber phases which necessarily occur during the consolidation of the woven material into a non-porous product.
An alternative method for making a composite material which avoids the use of ceramic slurries is that disclosed in U.S. Pat. No. 4,666,645. In the process disclosed in that patent, chopped fibers of the reinforcing material and fibers of the matrix material are mixed and combined into a non-woven felt material which may then be consolidated into a dense composite. Alternatively, the method employs felts made of the reinforcing fibers alternating with felts formed of fibers of the matrix material, with the superimposed felts thereafter being compressed and heat-consolidated into a dense material. Again, however, the application of this method is limited to matrix materials which can be fiberized, and substantial fiber movement and matrix flow are required for dense consolidation which can damage the reinforcing fibers.
A particularly desirable design for a fiber-reinforced ceramic matrix composite for applications requiring strong, thin ceramic materials is a design wherein uniform layers of parallel-aligned monofilament fibers are disposed within the ceramic matrix material. This design imparts very high strength and stiffness to the composite in directions transverse to the reinforcing fiber layers, and could in theory provide the advantage of a high degree of homogeneity with respect to both fiber distribution and physical properties in the laminates.
Unfortunately, no method yet devised for manufacturing ceramic matrix composites, including the slurry dip and fiber winding process earlier described, is presently capable of permitting the manufacture of composites of this design with the requisite homogeneous structure and high density.
It is therefore an object of the present invention to provide a method for manufacturing ceramic matrix composites, and particularly ceramic matrix composites employing uniform layers of parallel-aligned fiber reinforcement, which avoids the need to utilize a fiber coating process yet provides a composite of high density and extremely uniform microstructure.
It is a further object of the invention to provide a method for making a fiber-reinforced ceramic matrix composite, and particularly a composite comprising discrete layers or plies of uniaxially oriented fibers, which is both convenient and more economical than prior art methods for making such composites.
Other objects and advantages of the invention will become apparent from the following description thereof.